1. Field of the Invention
The present invention relates to a production control system, or more in particular to an apparatus, equipment and a control system required for constructing a flexible production system.
2. Description of the Prior Art
In conventional production control systems, an operating schedule is formed on the basis of a product specification for each piece of production equipment, equipment control data for implementing a job for each piece of equipment is prepared and stored in the related equipment, and the job is performed according to the operating schedule. These conventional systems require a system shutdown to correct equipment control units and the operating schedule to alter the product specification or the production equipment. A similar system shutdown and repair work is also required when fault of an equipment part occurs.
A solution to these problems has been suggested by “Production Control System of Process-Led Operation Selection Type” disclosed in JP-A-1-321503 and “Information Processing System for Processing Cells of Autonomous and Decentralized Type” in JP-A-2-236604, in which the equipment is not assigned with any job but autonomously selects and proceeds with a job for the work to thereby construct a flexible production control system.
According to the prior art disclosed in JP-A2-236604 primarily intended for a processing job, for example, each working cell making up a production control system conducts the job on its own as required on the basis of the processing information, etc. added to the work charged thereto. Also, a plurality of working cells having substantially the same specification are arranged to provide system flexibility, and makes possible the operation of a replacement cell when a cell malfunctions while at the same time assuring load equalization among different cells.
In the prior art systems described above, the equipment simply receives an incoming work and decides whether or not it is capable of performing the next job on the particular work. More specifically, if a job is capable of being performed, the equipment never fails to implement the next job. This system necessitates information exchange only with a single work in selecting a job, and therefore the algorithm for job selection is simple and causes no interference with the remaining equipment. In view of the fact that the equipment concentrates on a single work for job selection, however, the information regarding the remaining works is not taken into consideration in job selection. As a result, the delivery time of a particular product is disregarded. This not only makes it difficult to predict the completion time of a product, but also no priority is given even to a work of urgent necessity.
Where there are a plurality of works intended for a given job, the job is conventionally performed on first-come-first-served basis but no consideration is given to the order of priority based on delivery time or the like. As a consequence, a work with a short lead time, or especially, a work whose processing is of urgent necessity is unavoidably placed in the hands of the operator. Further, since works are controlled by work type, an urgent work which may occur cannot be processed until completion of a job on all the works of a particular work type in the process of operation.
In conventional systems, the processing operations required for works are similar to each other, and therefore flexibility could be simply obtained by a parallel arrangement of a plurality of cells of the same specification. In such production control systems, the workability including the possibility of a given job in other cells and the time required for the job are known in advance and need not be confirmed.
Nevertheless, in the assembly work which unlike the processing work has a smaller similarity between the jobs performed on different works, a system is difficult to construct only with cells of substantially the same specification. A production control system including the assembly work, therefore, requires an arrangement of cells having different functions. This requirement is not fully met by conventional production control systems which fail to confirm the workability of other cells as described above.